Could the MythBusters Shoot a Cannonball Made of Ice?

You can shoot all sorts of things from a cannon, but can you shoot a ball made of ice? The MythBusters set out to test that myth in their next episode, but let's first think about the physics of an ice cannonball.

Why Wouldn't It Work?
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What's the difference between an iron cannonball and one made of ice? Sure, they would have different masses (ice has a density around 1 g/cm3 and the density of iron is around7 7.9 g/cm3). Is that it? No. The two materials also have different compressive strength. What is compressive strength? It's the maximum pressure a material can withstand before failing. What does "fail" mean? It could mean that the material cracks - this is probably what ice would do. Or it could mean non-elastic deformation like iron would probably do.

Why would there be pressure on the ice ball? This is basically how the cannon works. Expanding gas from the black powder pushes on the ball on just one side. This pressure produces a force which accelerates the cannonball. More pressure means more force and a greater final speed.

What About the Strength of Ice?
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If you try a google search for "compressive strength of ice", you don't immediately find any useful results. Apparently, there are many different types of ice. Well, maybe you already knew this. If you get ice out of your freezer at home or ice from the drink machine at a fast food place, they look different. In fact they are different. One of the major factors in the property of ice is the air. Some ice has tiny air bubbles. This makes it look different and also has an effect on the strength.

But there is no doubt that ice can be strong. If you have experienced bitter cold in your area, you might know someone with frozen pipes that burst this winter. Clearly, this ice is stronger than the metal pipes (yes, I know copper isn't the strongest metal).

I know of one "material" that does NOT have a high compressive strength - pumpkins. This is the exact same issue that the Punkin Chunkin machines have when they try to get the best range. In case you aren't aware of Punkin Chunkin, the basic idea is to shoot a pumpkin as far as you can. One type of pumpkin cannon uses compressed air to launch a pumpkin out of a tube (essentially a giant potato gun). Sometimes the launch pressure on the pumpkin exceeds the strength of that pumpkin. You know what happens then? Yes, you just get pumpkin goo out the end of the gun. Here are some more details about this from an older post.

MythBusters Sneak Peak
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Do you want to see what happens when the MythBusters test an ice cannonball? Of course you do. Check out this exclusive clip from this upcoming show.

What happens with even more black powder? Can the MythBusters build a cannon out of ice also? I guess you'll have to watch the show to find out. And then after the show, you could consider the following questions (yes, these are for homework).

Use video analysis (I recommend the free Tracker Video Analysis) and confirm the launch speed of the ice cannonball at around 1000 feet per second. Actually, you can probably just pause the video and count frames. Let add that I am always amazed that they can even get video of such a fast moving object. Very impressive.

What about air drag? Use a model for the air drag on a sphere to estimate how much the ball would slow down over the approximate 3 meters of motion that is shown. Can you now measure the horizontal acceleration? You will need to estimate the mass of the ice cannonball.

Estimate the length of the cannon barrel. Use this with the muzzle velocity to estimate the average force on the cannonball during launch. Now estimate the pressure on the cannonball to get a minimum values for the compressive strength.

Compare the kinetic energy of the ice cannonball to the amount of black powder used. Is there a way to estimate the energy stored in black powder (energy density).

Yes, some of these questions are tough. That's what makes them great homework questions - well, that and the fact that I don't know the answers.